On-off servomechanism controlled by a wheatstone bridge

ABSTRACT

A pair of semiconductive switching elements connected in backto-back relation with each other are energized by a first DC source connected to a bridge circuit including at least two adjustable-slider resistors in such a way that both of the switching elements are turned off when the bridge circuit is balanced while one of the switching elements is turned on when the bridge circuit is unbalanced. The semiconductive switching elements are connected to relays such that when one switching element is turned on, an object to be controlled is controlled through the relay and the bridge circuit is balanced.

United States Patent Uchida [451 Oct. 10,1972

ON-OF F SERVOMECHANISM CONTROLLED BY A WHEATSTONE BRIDGE Inventor: Michio Uchida, Yokohama, Japan Assignee: Ricoh Co., Ltd., Tokyo, Japan Filed: Jan. 15, 1971 Appl. No.; 106,845

Foreign Application Priority Data Jan. 24, 1970 Japan ..45/6490 U.S. Cl ..318/674, 318/663 Int. Cl. ..G05b 11/14 Field of Search ..3 1 8/663, 674

References Cited UNITED STATES PATENTS 2/1961 Montross ..3 18/674 3,241,047 3/1966 McCabe ..3 18/663 Primary Examiner-T. E. Lynch Attorney-Henry T. Burke, Robert Scobey, Robert S. I Dunham, P. E. Henninger, Lester W. Clark, Gerald W. Griffin, Thomas F. Moran, R. Bradlee Boal and Christopher C. Dunham 5 7 ABSTRACT A pair of semiconductive switching elements connected in' back-to-back relation with each other are energized by a first DC source connected to a bridge circuit including at least two adjustable-slider resistors in such a way that both of the switching elements are turned off when the bridge circuit is balanced while one of the switching elements is turned on when the bridge circuit is unbalanced. The semiconductive switching elements are connected to relays such that when one switching element is turned on, an object to be controlled is controlled through the relay and the bridge circuit is balanced.

4 Claims, 3 Drawing Figures PATENTEDBBT 101112 3.69? 849 sum 1 or 2 M/m/o 1/09/04 I N V E N To R BY flaw M ATTORNEY BACKGROUND OF THE INVENTION The present invention relates to a servomechanism and more particularly a simple and inexpensive servomechanism best adapted for use in a photocopying machine or the like for shifting a zoom lens or for use in an offset printing press for stepless machine driven'with V-belt.

In the case of servomechanisms'used in conjunction with profiling machines, automatically balanced instruments and the like, high degree of accuracy, stability, quick response and the like-are important factors in design andoperation. Systems which are adequate in terms of these factors areoften very complex in construction. Such machines require very sensitive servoamplifiers, and servomotors having low inertia and having torque which is reduced as speed is reduced. In some cases, generators are required. However, when a zoomlens of a photocopying. machine is shifted, or when a heavy load is displaced by means of a stepless speed change machine driven with V-belt in an offset printing press by an electric motor through the manual operation of an adjustable resistor, the importance of factors such as accuracy, response, stability and the like of the servomechanism is reduced. Such servomechanism need improvements only in terms of simplificationof construction and reduction in cost.

SUMMARY OF THE INVENTION One of the objects of the present invention is to provide a servomechanism in which the direction of current flow may be controlled, by semiconductive switching elements such as silicon controlledrectifiers or transistors without the use of specially designed servoamplifiers or servomotors, so as to rotate a motor in a selected direction.

Another object of the present invention is to provide a simple and inexpensive servomechanism.

In brief, a servomechanism in accordance with the present invention includes a bridge circuit having at least two adjustable-slider resistors, two semiconductive switching elements connected in back-to-back relation with each other and connected to said bridge circuit in such a way that when the bridge circuit is balanced the switching elements are turned off while when the bridge circuit is not balanced one of the switching elements is turned on, and two relays connected in series to the output terminals of the switching elements respectively. The switching elements are energized by a first DC circuit while'the bridge circuit is fed with the current from a second DC source. The control electrode of one of the switching elements is electrically connected through a resistor to the sliding arm of one of the adjustable-slider resistors in the bridge circuit while the control electrode of the other switching element is connected electrically to the sliding arm of the other adjustable-slider resistor. When the bridge circuit is unbalanced, one of the relays connected to one of the switching elements is actuated to close a motor driving circuit so as to drive a means for moving the slider arm of one of the adjustable-slider resisters and a device for displacing or actuating an object to be controlled. The motor is stopped when the motor-driven slider arm of the adjustable-slider resistor ed h spe cange m is moved to a point at which the bridge circuit is balanced so that the object to be controlled is stopped.

The present invention will become more apparent from the following description of the preferred embodiments thereof taken in conjunction with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a circuit diagram of one embodiment of the present invention;

FIG. 2 is a circuit diagram the present invention; and

FIG. 3 is a circuit diagram of a specific embodiment of the present invention as used for shifting a zoom lens in a photocopying machine.

DESCRIPTION OF THE PREFERRED EMBODIMENTS of another embodiment of I described in connection with FIG. 3) may be connected to an adjustable slider resistor 20 such that when another adjustable-slider resistor 18 is manually operated, the adjustable-slider resistor 20 is driven by the motor. Alternately, the connection may besuch that when the adjustable-slider resistor 20 is manually operated, the resistor 18 is driven by the motor. The reference numerals 22, 24, 30 and 32 designated resistors; 26 and 28 designate capacitors; and 27 and 29 designate diodes for absorbing the back or inverse current of the relays l4 and 16 respectively.

When the potential at the point labeled A is equal to that at the point B, that is when the circuit is balanced, no current flows into the gates of the SCRs 10 and 12, so that they are non-conducting. Consequently the relays l4 and 16 are kept opened. It is assumed that the adjustable resistor 18 is manually operated while the adjustable resistor 20 is controlled by the motor. When the slider of the adjustable resistor 18 is moved so as to increase the voltage at point A, the voltage across the points A and C becomes higher than that across the points B and D. The potential at the point A becomes higher than that at the point B and current flows into the gate of the SCR 10 so that it is turned on, thereby energizing the relay 14. In consequence, the motor is driven so as to move the slider of the adjustable resistor 20 so as to increase the voltage across the points 8 and D. Therefore, the voltage difference between the points A and B approaches zero. At the instant when the voltage difference between the points A and B becomes zero, the SCR 10 is turned off to thereby de-energize the relay 14.50 that the motor is stopped.

On the other hand, when the slider of the adjustable resistor 18 is manually moved so as to reduce the voltage across the points A and C, the voltage across the points A and C becomes lower than that across the points B and D. The potential at the point B becomes higher than that at the point A so that current flows into the gate of the SCR 12 to energize thereby the relay 16 so that the motor is driven. Upon rotation of the motor, the slider of the adjustable resistor 20 is moved so as to reduce the voltage across the points B and D so that the voltage difference between the points A and B approaches zero. At the instant when the voltage difference between the points A and B becomeszero, the SCR 12 is turned off so that the relay 16 is deenergized, thereby stopping the motor.

The output of the DC source 1 must be half-wave or full-wave rectified because it is used to turn off the SCRs l and 12 when their gate currents are zero. The output of the second DC source 2 may be of any waveform. Both of the output voltages E, and E may be arbitrarily selected as far as the relays 14 and 16 are actuated in a normal manner at a voltage range between (E -E and E,. In order to increase the sensitivity or control ability for controlling the ON-OFF operations of the SCRs l0 and 12 in response to the movement of the slider of the adjustable resistor 18, the output voltage E can be of the order of tens of volts, while the output voltage, E can be somewhat higher than 100 volts when the relays l4 and 16 are of about 100 volt rating. If a high sensitivity is not required, the output voltage E may be of the order of tens of volts while the output voltage E may be a few volts.

The values of the resistors 18 and 20 are very small as compared with the resistance of the coils of the relays 14 and 16 so that no excessive transients are introduced in the currents through the resistors 18 and 20 because of changes between ON and OFF of the SCRs and 12.

The gate currents of the SCRs 10 and 12 are controlled by the fixed-value resistors 22 and 24, whose values influence the sensitivity of the servomechanism. That is, the sensitivity is reduced when the values of the resistors 22 and 24 are increased while the sensitivity becomes too high when the values are so low that oscillation occurs due to the inertia of the motor and the load. The preferable value is about 1 kilo-ohm.

The capacitors 26 and 28 in series with the fixed resistors 30 and 32, respectively, are for reshaping the waveforms to smooth the voltages applied across the coils of the relays l4 and 16. When the SCR 10 or 12 is turned on so as to actuate the relay 14 or 16, the capacitor 26 or 28 serves to prevent the distortion of the waveforms of the voltages across the points B and D or A and C. For this purpose, the values of the fixed resistors 30 and 32 must be suitably higher than those of the adjustable-slider resistors 18 and 20.

Another embodiment of the present invention, illustrated in FIG. 2, is substantially similar to the embodiment of FIG. 1, except that transistors 10a and 12a are utilized in place of the SCRs l0 and 12. The mode of operation is similar to that of the first embodiment. The breakdown voltages of the transistors 10a and 12a are lower than those of the SCRs l0 and 12, so that the output voltages of the DC sources 1 and 2 must be reduced accordingly. In general, since the switching characteristic of silicon controlled rectifiers is better than that of transistors, it is advantageous in most cases to use silicon controlled rectifiers rather than transistors.

A specific embodiment of the present invention is illustrated in FIG. 3, in which the component parts similar to those in FIG. 1 are designated by the same reference numerals. The slider arm of the resistor 18 may be moved by a knob 3, and a reversible singlephase induction motor 6 may be driven by an AC power source 4 through a capacitor 5 so that the torque of the motor 6 may be transmitted to a timing puller 7, which in turn drives a timing belt 8 carrying a zoom lens housing 9. The movement of the zoom lens housing 9 is supported by a support 13 in engagement with a guide rail 11. The adjustable-slider resistor 20 is coupled to the rotary shaft of the motor 6.

To move the zoom lens housing 9 to a desired position to the right of the present position, the dial or knob 3 is rotated through a predetermined angle in the counterclockwise direction so that the relay 14 is actuated. In consequence the contact 14a of the relay 14 is closed to drive thereby the motor 6, for example in the counterclockwise direction. The zoom lens housing 9 is moved toward the right by means of the pulley 7 and the belt 8 while the slider arm or wiper of the adjustable resistor 20 is moved in the direction increasing the voltage across the points B and D. When the lens housing 9 reaches the desired position, the voltage across the points B and D becomes equal to that across the points A and C. That is, the voltage difference across the points A and B becomes zero. Then the relay 14 is actuated to open thereby the contact 14a to stop the motor 6 and hence the housing 9. g

In order to move the housing 9 to a position to the left of the present position, the dial or knob 3 is rotated in the clockwise direction. In this case, the relay 16 is actuated so as to drive the motor 6 in the clockwise direction. When the voltage across the points B and D becomes equal to that across the points A and C, that is when the voltage difference between the points A and B becomes zero, the relay 16 opens the contact 1611 to stop the motor 6 and hence the housing 9 at the desired position.

It is clear to those skilled in the art that the angle of rotation of the dial or knob 3 may be correlated with the position at which the lens housing 9 is desired to be stopped by interposing a gear train or belt (not shown explicitly in FIG. 3) between the motor 6 and the pulley 7 In the embodiments described above, two adjustableslider resistors of identical construction are used to form a bridge for reasons of economy but it is understood that a bridge may be formed with two fixedvalue resistors and two adjustable-slider resistors. In addition, an AC or DC motor may be used as far as it is reversible. In order to prevent variations in sensitivity due to temperature changes, suitable diodes may be inserted in the circuit as compensators.

In summary, the servomechanism of the present invention is simple and economical in construction and is best adapted when stability, quick response and a higher degree of accuracy are not particularly important factors.

Iclaim:

1. A photocopying machine having a zoom lens housing (9) and supporting means (7, 8, 11, 13) therefor, wherein the improvement is in means for moving the zoom lens housing (9), comprising:

a. a first DC source (1),

b. a second DC source (2),

c. a bridge circuit which includes at least two adjustable-slider resistors (18, 20) with slider arms and which is connected to said second DC source (2) to receive current therefrom,

d. two semiconductive switching elements (10, 12)

e. -f. two relays (l4, 16), each connected in series with a each having a gate terminal and two main terminals, and each being normally an open circuit between its main terminals but changing to a closed circuit therebetween when turned ON, which elements (10, 12) are connected in back-toback relation with each other, with the gate terminal of each element (l0, 12) connected to a main terminal of the other element through a voltage dropping resistor (22, 24), and which elements (10, 12) are connected to said first DC source (1) and to said bridge circuit, both of said semiconductive switching elements (l0, 12) being turned OFF when the bridge is balanced and one of said semiconductive switching elements (10, 12) being turned ON when the bridge is unbalanced,

a motor (6), I

main terminal of one of said semiconductive switching elements (10, 12) to be turned on and off in response to the ON and OFF state respectively of the semiconductive switching element (l0, 12) with which it is connected, said relays (14, 16) being connected to the motor (6) to drive the motor in one direction when one of said relays (14, 16) is turned on and to drive the motor (6) in the opposite direction when the other replay (l4,

16) is turned on,

g. means (3) for manually unbalancing the bridge,

and

h. a zoom lens housing (9) and means (7, 8, ll, 13)

supporting the zoom lens housing, said motor being drivingly connected to the slider arm of one of said adjustable-slider resistors and to the supporting means (7, 8, 11, 13) of the zoom lens housing (9) to drive both the slider arm and the zoom lens housing (9) when the bridge circuit is unbalanced until said bridge circuit is balanced and to move thereby the zoom lens housing (9) toward a desired position.

2. A photocopying machine as in claim 1 wherein said first DC source is selected from the group consisting of a half-wave rectified current DC source and a full-wave rectified current DC source.

3. A photocopying machine as in claim 1 wherein said semiconductive switching elements are siliconcontrolled rectifiers.

4. A photocopying machine as in claim 1 wherein said semiconductive switching elements are transistors. 

1. A photocopying machine having a zoom lens housing (9) and supporting means (7, 8, 11, 13) therefor, wherein the improvement is in means for moving the zoom lens housing (9), comprising: a. a first DC source (1), b. a second DC source (2), c. a bridge circuit which includes at least two adjustableslider resistors (18, 20) with slider arms and which is connected to said second DC source (2) to receive current therefrom, d. two semiconductive switching elements (10, 12) each having a gate terminal and two main terminals, and each being normally an open circuit between its main terminals but changing to a closed circuit therebetween when turned ON, which elements (10, 12) are connected in back-to-back relation with each other, with the gate terminal of each element (10, 12) connected to a main terminal of the other element through a voltage dropping resistor (22, 24), and which elements (10, 12) are connected to said first DC source (1) and to said bridge circuit, both of said semiconductive switching elements (10, 12) being turned OFF when the bridge is balanced and one of said semiconductive switching elements (10, 12) being turned ON when the bridge is unbalanced, e. a motor (6), f. two relays (14, 16), each connected in series with a main terminal of one of said semiconductive switching elements (10, 12) to be turned on and off in response to the ON and OFF state respectively of the semiconductive switching element (10, 12) with which it is connected, said relays (14, 16) being connected to the motor (6) to drive the motor in one direction when one of said relays (14, 16) is turned on and to drive the motor (6) in the opposite direction when the other replay (14, 16) is tuRned on, g. means (3) for manually unbalancing the bridge, and h. a zoom lens housing (9) and means (7, 8, 11, 13) supporting the zoom lens housing, said motor being drivingly connected to the slider arm of one of said adjustable-slider resistors and to the supporting means (7, 8, 11, 13) of the zoom lens housing (9) to drive both the slider arm and the zoom lens housing (9) when the bridge circuit is unbalanced until said bridge circuit is balanced and to move thereby the zoom lens housing (9) toward a desired position.
 2. A photocopying machine as in claim 1 wherein said first DC source is selected from the group consisting of a half-wave rectified current DC source and a full-wave rectified current DC source.
 3. A photocopying machine as in claim 1 wherein said semiconductive switching elements are silicon-controlled rectifiers.
 4. A photocopying machine as in claim 1 wherein said semiconductive switching elements are transistors. 